If you insert your iPhone's headset into your iPhone, and then plug an iPhone to USB connector into the bottom connector, the phone has a very useful set of behaviors. Regular audio such as music or podcasts will go out the bottom connector—which in my case ends up coming out my car's speakers—while phone conversations will go through the headsets. Also, the click button on the microphone works to pause (single click), answer (single click), hang up (single click) and skip to next track (double click).
Therefore, when I get into the car for my commute home, I plug my headset into the phone, put the right—microphone—ear bud into my ear, and then plug the phone into my car's stereo system. The order this is done is important. Then, as I'm driving around, if someone calls me, I can answer the phone, hang up, and skip tracks all without looking. Be warned, might be illegal in your locality.